C#数据结构与算法系列（十）：中缀表达式转后缀表达式

 /// <summary>
        /// 字符串转中缀表达式的List
        /// </summary>
        /// <param name="expression"></param>
        /// <returns></returns>
        public static List<string> ToInfixExpression(string expression)
        {
            List<string> list = new List<string>();

            int index = 0;

            string str = "";

            do
            {

                //48-57ASCII码代表的是0-9 如果不是0-9直接入链表
                if (expression[index] < 48 || expression[index] > 57)//ascii编码
                {
                    list.Add("" + expression[index]);

                    index++;
                }
                else
                {
                    str = "";

                    //多位数判断
                    while (index < expression.Length && expression[index] >= 48 && expression[index] <= 57)
                    {

                        str += expression[index];

                        index++;
                    }

                    list.Add(str);
                }

            } while (index < expression.Length);

            return list;
        }

    /// <summary>
        /// 中缀转后缀
        /// </summary>
        /// <param name="expression"></param>
        /// <returns></returns>
        public static List<string> ParseSuffixExpression(List<string> expression)
        {
            //存储中间结果
            List<string> list = new List<string>();
            //符号栈
            Stack<string> stack = new Stack<string>();

            foreach (var item in expression)
            {
                //多位数判断 如果是数字直接加入list
                if (Regex.IsMatch(item, "\\d+"))
                {
                    list.Add(item);
                }
                //如果是左括号，直接入符号栈
                else if (item.Equals("("))
                {
                    stack.Push(item);
                }
                //如果是右括号
                else if (item.Equals(")"))
                {
                    //依次弹出stack栈顶的运算符，并存入list,直到遇到左括号为止
                    while (!stack.Peek().Equals("("))
                    {                        
                        list.Add(stack.Pop());
                    }
                    //将(也出栈
                    stack.Pop();
                }
                //如果是*/+-
                else
                {
                    //循环判断item的优先级小于或者等于stack栈顶运算符，将stack栈顶的运算符出栈并加入到list中
                    while (stack.Count != 0 && Operation.GetValue(stack.Peek()) >= Operation.GetValue(item))
                    {
                        list.Add(stack.Pop());
                    }
                    //将item入栈
                    stack.Push(item);
                }
            }

            //将stack剩余的运算符依次入list
            while (stack.Count!=0)
            {
                list.Add(stack.Pop());
            }

            return list;
        }

    public class Operation
    {
        private static int ADD = 1;
        private static int SUB = 1;
        private static int MUL = 2;
        private static int DIV = 2;

        public static int GetValue(string operation)
        {
            int result = 0;

            switch (operation)
            {
                case "+":
                    result = ADD;
                    break;
                case "-":
                    result = SUB;
                    break;
                case "*":
                    result = MUL;
                    break;
                case "/":
                    result = DIV;
                    break;
                default:
                   // Console.WriteLine("不存在该运算符");
                    break;
            }

            return result;
        }
    }

        /// <summary>
        /// 计算
        /// </summary>
        /// <param name="list"></param>
        /// <returns></returns>
        public static int Calculate(List<string> list)
        {
            //创建栈
            Stack<string> stack = new Stack<string>();

            //循环遍历
            list.ForEach(item =>
            {
                //正则表达式判断是否是数字，匹配的是多位数
                if (Regex.IsMatch(item,"\\d+"))
                {
                    //如果是数字直接入栈
                    stack.Push(item);
                }
                //如果是操作符
                else
                {
                    //出栈两个数字，并运算，再入栈
                    int num1 =int.Parse(stack.Pop());

                    int num2 = int.Parse(stack.Pop());

                    int result = 0;

                    if(item.Equals("+"))
                    {
                        result = num2 + num1;
                    }
                    else if(item.Equals("*"))
                    {
                        result = num2 * num1;
                    }
                    else if(item.Equals("/"))
                    {
                        result = num2 / num1;
                    }
                    else if (item.Equals("-"))
                    {
                        result = num2 - num1;
                    }
                    else
                    {
                        throw new Exception("无法识别符号");
                    }

                    stack.Push(""+result);
                }
            });

            //最后把stack中数据返回
            return int.Parse(stack.Pop());
        }

    public class ReversePolandTransformation
    {

        public static void Test()
        {
            string expression = "1+((2+3)*4)-5";

            //将字符串转换成List
            List<string> infixExpression = ToInfixExpression(expression);

            string str = "";

            infixExpression.ForEach(item =>
            {
                str = str + item + ",";
            });

            Console.WriteLine("中缀表达式对应的List:"+str);

            str = "";

            //将中缀表达式转换成后缀表达式
            List<string> suffixExpression = ParseSuffixExpression(infixExpression);

            suffixExpression.ForEach(item =>
            {
                str = str + item + ",";
            });

            Console.WriteLine("\n后缀表达式对应的List："+str);

            //结果计算
            int result =PolandNotation.Calculate(suffixExpression);

            Console.WriteLine($"\n{expression}={result}");
        }
   }